Um... the same people who collect it now? 45 states plus DC already have sales taxes -- the mechanism is already there.
Um.... don't hold your breath. You think all of those IRS agents are just going to clean out their desks and go get an honest job without a fight? If you were a retailer, think about what the ratio of IRS maggots to retailers will be. And when that "retail" sales tax gets extended to services, every small business in america will have their own personal IRS agent.